1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for compressing a fluid.
2. Discussion of the Background
During the past years, the demand for various chemical products has increased. One such example is the demand for polyethylene and polypropylene products, which are used, for example, in the plastics industry, the multiphase pipelines industry, etc. The manufacturing of these products has consequently also increased. One of the mechanical components used in a plant (reactor) for producing the polyethylene or polypropylene products is a centrifugal compressor.
Compressors are generally divided into positive displacement compressors and dynamic compressors. Positive displacement compressors include reciprocating and rotary compressors, which are not discussed herein. Dynamic compressors include, among others, centrifugal compressors, axial compressors and mixed-flow compressors.
An example of a centrifugal compressor is shown in FIG. 1. FIG. 1 shows the centrifugal compressor 10 having an impeller 12 connected to a shaft 14. Shaft 14 is supported by bearings 16 and 18. The impeller 12 has a hub portion 20 and a blade portion 22. A fluid enters the compressor 10 at an inlet 24, along an incoming direction A. The fluid reaches the impeller 12, where its kinetic energy is increased and its flow direction is changed prior to being discharged at outlet 26 along direction B. Because the impeller 12 is not supported on shaft 14 between bearings 16 and 18, this arrangement is called “overhung” compressor as distinct from a “between-bearings” design where the impeller(s) are supported between the bearings. In addition, because a centrifugal force produced by the impeller 12 is used to accelerate the fluid entering the compressor 10, the compressor shown in FIG. 1 is called an overhung centrifugal compressor.
The overhung centrifugal compressor is widely used in the chemical and petrochemical industry. However, a disadvantage of this compressor is its large size for a given set of processing parameters, e.g., flow parameters. For example, FIG. 2 shows a graph of a head coefficient of a compressor versus its flow coefficient. The head coefficient is related to an output pressure of the compressor and it is a dimensionless coefficient. The flow coefficient is related to a volume flow rate of the fluid through the compressor. FIG. 2 shows a variation in time of the head coefficients and flow coefficients of the existing compressors developed for the polyethylene/polypropylene industry, with the points to the left being earlier in time then the points in the right. This graph indicates that smaller head coefficients and larger flow coefficients have been required by the operators of the plants over time. Following this trend, the weight of the centrifugal compressors (especially the casing) has increased in the past ten years from an average of 20 tons to an average of 40 tons with a diameter of the impeller increasing from 45 cm to over 90 cm. By increasing the weight and size of the compressors, the weight and size of associated components, i.e., diffuser, etc., has also increased.
Another drawback of the centrifugal compressor is the fact that a, polytropic efficiency of the compressor decreases as the flow coefficient is increased beyond a certain point. Mixed flow compressors have been used for addressing the deficiencies of the centrifugal compressors as the flow coefficient become too large. However, these compressors are also reaching their limits in terms of efficiency and desirable weight and are presently suffering from the same problems as the centrifugal compressors. The mixed flow compressors are similar to the centrifugal compressors but the fluid is expelled at an angle with respect to a longitudinal axis of the compressors. In other words, the direction of the outgoing fluid is between directions A and B shown in FIG. 1, being neither axial flow (direction A) nor radial flow (direction B).
FIG. 3 shows the variation of impeller diameter (for a mixed flow compressor) versus the rotational speed (curve 30) for a given polytrophic head requirement. Also, the flow coefficient is plotted versus the impeller rotational speed (curve 32) for a given flow requirement. It is noted that for a 41-in diameter impeller (point 34) the corresponding flow coefficient is around 0.172 (point 36), which is in the generally desired range of less than approximately 0.25 for mixed-flow impellers. However, when trying to reduce the size of the impeller to around 27-in (point 38), which is approximately a 35% reduction in size, the flow coefficient goes up to 0.4, which is outside the desired range for good efficiency.
An axial compressor is illustrated in FIG. 4. The axial compressor 42 has a shaft 44 to which plural airfoils 46 are connected. A fluid enters inlet 48 and is accelerated through the plural airfoils 46, along an axial direction C, until the fluid is expelled at outlet 50. However, due to dirt particles in the fluid, deposits may be formed on the airfoils 46 and on the casing 52 of the compressor 42. For cleaning the airfoils and the casing, an upper part (not shown) of the compressor is removed for accessing the parts to be cleaned. This split of the casing 52 of the axial compressor along a horizontal plane makes this compressor a horizontal split casing axial compressor. Also, the typical axial compressor has both ends 54 of shaft 44 supported by bearings and the airfoils 46 are disposed between the bearings supporting shaft 44.
The axial compressor achieves a better flow coefficient and a smaller size impeller (airfoils) than the centrifugal and/or mixed flow compressors, and thus, a smaller weight and size. However, the drawback with the existing axial compressors is the difficulty in maintaining the axial compressor if used under dirty process gas conditions, as found in the polyethylene/polypropylene industry, as the airfoils become clogged and opening up the axial compressor and cleaning its components become time consuming and expensive.
Accordingly, it would be desirable to provide compressors, and methods that avoid the afore-described problems and drawbacks.